|
|
In double replacement reactions, reactant cations have been interchanged to form products. One example is the reaction between aqueous barium chloride, BaCl2, and aqueous sodium sulfate, Na2SO4. The positive ions trade places to form solid barium sulfate, BaSO4, and salt, NaCl. Both reactants and products are compounds. Notice how the polyatomic sulfate ion stays intact while the cations are exchanged. 
|
|
|
Double replacement, also known as double displacement reactions often form a precipitate. A precipitate is a solid that forms when reactant ions in solution produce an insoluble compound, meaning it cannot dissolve. Look for a solid barium sulfate precipitate in the reaction above. The insoluble compound may settle out on the bottom of a beaker, or may remain suspended. A suspended precipitate turns a solution from clear to cloudy.
|
|
To predict products of a double replacement reaction, write ions for both reactants. Trade cation places and criss-cross charges to determine the products formed. Cations always come first in an ionic compound, so only the first ion in each compound will trade places.
|
| Write the balanced equation for the reaction of lead(II) nitrate with potassium sulfide.
|
| Relationships |
Two compounds are reacting; this must be double replacement. You must exchange cations to determine products. |
| Solve |
- Write the ions and criss-cross charges to determine reactant formulas:
- Ions in lead(II) nitrate: Pb2+ and NO3−
- Formula after criss-cross: Pb(NO3)2
- Ions in potassium sulfide: K+ and S2-
Formula after criss-cross: K2S
- Start writing the reaction: Pb(NO3)2 + K2S → ? + ?
- Write new formulas for each cation by pairing it with a new anion.
- For lead (II):
- Write ions that will combine: Pb2+ and S2-
- Criss-cross charges; in this case both charges cancel: PbS
- For potassium:
- Write ions that will combine: K+ and NO3−
- Criss-cross charges: KNO3
- Write the reaction: Pb(NO3)2 + K2S → PbS + KNO3
- Balance the reaction: Pb(NO3)2 + K2S → PbS + 2KNO3
|
| Answer |
The balanced reaction is: Pb(NO3)2 + K2S → PbS + 2KNO3 |
|
| |
|
